An electromagnetic actuator comprises at least one armature and, for example, two coils. The actuator can also comprise two permanent magnets. The position of the armature in the coils produces various inductances. In a method for activating the actuator, for example during an abruptly increasing energizing current, the voltage variation at the two coils can be measured. From these measurement data it is possible, for example in a differentiator, to compute a third voltage variation or a sensor signal from which a logic unit can determine the position of the armature. Thus, the position can be determined by evaluating the inductances prevailing at the coils.
In ferromagnetic materials the relationship between magnetic field strength H and flux density B is non-linear and depends on the previous history. Responsible for this are magnetic flux densities that influence the inductance and hence the sensor signal. The magnetic flux density in ferromagnetic components depends on the strength of the prevailing magnetic field. If the prevailing magnetic field decreases, ferromagnetic components have some residual magnetism. Depending on the direction and strength of the previously applied magnetic field, the flux density of the residual ferromagnetism can vary. Since the magnetic flux density has to be evaluated, it is accordingly possible for a position signal to be obtained, which is made erroneous owing to different previous histories. This effect is not desired in measurement instruments and sensors, but it occurs frequently, particularly when magnetic systems are part of the measurement value receiver. Furthermore, temperature fluctuations influence the residual magnetism remaining and thus, in turn, the position detection.
WO 2010/049200 A1 relates to a method for detecting the position of a magnetic armature of an electromagnetic actuator arranged between two coils.